The invention relates generally to power sources, and one embodiment of the invention may be used in an implantable medical device. Implantable power sources for medical devices are not new. Many such devices use a rechargeable cell to provide power, and these cells are often lithium-ion cells. The high energy density of a lithium-ion cell results in the dimensions of the power source being smaller than for power sources using different cell-chemistry.
In order to achieve high voltage output, cells of prior art devices are connected in series to a load, such as a defibrillator, left ventricular assist pump, hearing aides or a total artificial heart. When the cells need to be recharged, the load is electrically disconnected from the cells, and a charging power source is electrically connected to the cells. The series-connected cells are then charged, often by a constant current and then by a constant voltage, until they reach a predetermined voltage. Due to the high energy density of lithium-ion cells, the charge and discharge processes must be performed according to precise guidelines. Once charged, the load is re-connected to the cells.